Transcutaneous electrical nerve stimulation (TENS) is the delivery of electricity (i.e., electrical stimulation) across the intact surface of a user's skin in order to activate sensory nerve fibers. The most common application of TENS therapy is to provide analgesia, such as for chronic pain. Other applications of TENS therapy include, but are not limited to, reducing the symptoms of restless leg syndrome, decreasing nocturnal muscle cramps, and providing relief from generalized pruritis. A conceptual model for how sensory nerve stimulation leads to pain relief was proposed by Melzack and Wall in 1965. Their theory stipulates that activation of sensory nerves (Aβ fibers) closes a “pain gate” in the spinal cord that inhibits the transmission of pain signals carried by nociceptive afferents (C and Aδ fibers) to the brain. In the past 20 years, anatomic pathways and molecular mechanisms that may underlie the pain gate have been identified. Sensory nerve stimulation (e.g., via TENS) activates the descending pain inhibition system, primarily the periaqueductal gray (PAG) and rostroventral medial medulla (RVM) located in the midbrain and medulla sections of the brainstem, respectively. The PAG has neural projections to the RVM, which in turn has diffuse bilateral projections into the spinal cord dorsal horn that inhibit ascending pain signal transmission.
TENS is typically delivered in short discrete pulses (with each pulse typically being several hundred microseconds in duration) at frequencies between about 10 and 150 Hz, through hydrogel electrodes placed on the user's body. TENS is characterized by a number of electrical parameters including the amplitude and shape of the stimulation pulse (which combine to establish the pulse charge), the frequency and pattern of the pulses, the duration of a therapy session and the interval between therapy sessions. All of these parameters are correlated to the therapeutic dose. For example, higher amplitude and longer pulses (i.e., larger pulse charge) increase the dose, whereas shorter therapy sessions decrease the dose. Clinical studies suggest that pulse charge and therapy session duration have the greatest impact on therapeutic dose.
To achieve maximum pain relief (i.e., hypoalgesia), TENS needs to be delivered at an adequate stimulation intensity. Intensities below the threshold of sensation are not clinically effective. The optimal therapeutic intensity is often described as one that is “strong yet comfortable”. Most TENS devices rely on the user to set the stimulation intensity, usually through a manual intensity control comprising an analog intensity knob or digital intensity control push-buttons. In either case (i.e., analog control or digital control), the user must manually increase the intensity of the stimulation to what the user believes to be a therapeutic level. Therefore, a major limitation of current TENS devices is that it may be difficult for many users to determine an appropriate therapeutic stimulation intensity. As a result, the user will either require substantial support from medical staff or they may fail to get pain relief due to an inadequate stimulation level.
A newly-developed wearable TENS device (Quell®, Neurometrix, Inc., Waltham, Mass., USA) uses a novel method for calibrating the stimulation intensity in order to maximize the probability that the TENS stimulation intensity will fall within the therapeutic range. With the Quell® device, the user identifies their electrotactile sensation threshold and then the therapeutic intensity is automatically estimated by the TENS device based on the identified electrotactile sensation threshold.
Pain relief from TENS stimulation usually begins within 15 minutes of the stimulation onset and may last up to an hour following the completion of the stimulation period (also known as a “therapy session”). Each therapy session typically runs for 30-60 minutes. To maintain pain relief (i.e., hypoalgesia), TENS therapy sessions typically need to be initiated at regular intervals. Newly-developed wearable TENS devices, such as the aforementioned Quell® device, provide the user with an option to automatically restart therapy sessions at pre-determined time intervals.
The persistent nature of chronic pain and the convenience of “wear-and-forget” TENS technology may lead some users to wear the TENS device daily for an extended period of time. To achieve maximum pain relief, TENS needs to be delivered at an adequate stimulation intensity level throughout the day and also at night (i.e., when the user is asleep). The optimal therapeutic stimulation intensity level varies from person to person, and depends upon the electrotactile threshold of each individual user. Once the optimal setting for the therapeutic stimulation intensity level is determined for a particular user, it remains fixed for that user for all subsequent TENS therapeutic sessions throughout the day.
However, all organisms have internal “clocks” that regulate normal biological processes and normal physiological function. The most important and well understood internal “clock” is the circadian rhythm. In the absence of external entrainment cues, the human circadian rhythm has a 20 to 28 hour cycle. The circadian oscillator is synchronized to the physical 24-hour day-night cycle by environmental signals such as light. Therefore, a single time-invariant TENS dose may not provide consistent pain relief throughout the day for a TENS user.
A growing recognition of the importance of the circadian rhythm, and other temporal fluctuations, in various diseases and the efficacy of their treatments has led to the concept of “chronotherapy,” which is an attempt to design therapeutic approaches that account for the temporal properties of human physiological function. By way of example but not limitation, circadian rhythms influence chronic pain and may impact the treatment of pain using TENS therapy. Variations in pain intensity over the course of the day are common. Some pain conditions, such as painful diabetic neuropathy, exhibit peak intensity (i.e., the greatest level of pain) in the evening, while other pain conditions, such as fibromyalgia, exhibit peak intensity (i.e., the greatest level of pain) in the morning. One significant implication of these fluctuations in the degree of pain experienced by the user over the course of the day is that a user may require a higher therapeutic dose (i.e., a higher level of TENS stimulation) at certain times of the day in order to achieve optimal and stable pain control.
A user's sensory threshold may vary over the course of the day, which may also impact the efficacy of TENS therapy at a given stimulation intensity level. In other words, the threshold at which a sensory stimulus (e.g., electrical stimulation, light, heat, etc.) is detected by the user is not constant, but varies over the course of the 24-hour cycle. Although, circadian variation in the perception threshold to electrical stimulation, commonly referred to as the “electrotactile threshold”, has not been studied extensively, several published studies suggest that humans experience time-varying perception thresholds to electrical stimulation (e.g., TENS therapy). Most users experience their lowest perception threshold (i.e., greatest sensitivity) to electrical stimulation (e.g., TENS therapy) in the late afternoon and early evening (see, for example, Sheriden et al., “Some Factors Influencing the Threshold of the Electrocutaneous Stimulus”. Percept. Mot. Skills, 1966). However, there is substantial inter-individual variation and some users experience a minimum perception threshold at other times of the day. The implication of a varying electrotactile perception threshold is that the therapeutic effect of TENS stimulation therapy may vary in a circadian fashion if the stimulation intensity is held constant throughout the day. More particularly, if the user's electrotactile perception threshold is low, then more sensory nerves will be stimulated as compared to when the user's electrotactile perception threshold is high.
The anatomical location where circadian modulation occurs may be in the periphery of the user's body, in the user's central nervous system (CNS), or both. In the periphery of the user's body, modulation of nerve stimulation may be due to changes in body surface temperature, biophysical changes in peripheral nerve membranes, and other effects. Circadian rhythms may also modulate sensory perception in the CNS where the integration of peripheral sensory signals may be amplified or attenuated in a time-varying fashion. Regardless of the site(s) of circadian control/modulation of the electrotactile perception threshold, the net effect is that the sensory input that triggers the descending pain inhibition system fluctuates in a rhythmic fashion, leading to an oscillation in the effective stimulation intensity. To maintain stable and uniform therapeutic effectiveness of TENS therapy for a particular user, the circadian rhythms of that particular user can be exploited in order to optimally regulate TENS stimulation parameters, with the goal of enhancing TENS therapeutic effectiveness by counteracting the time-dependent nature of the sensory perception threshold and pain level.